It has been known for a relatively long time that dielectric (non-metallic) materials can be heated by applying energy in the form of microwaves. The origin of such heating is derived from the ability of the electric field to polarise charges in the material and the inability of this polarisation to follow rapid reversals of the external electromagnetic field. The ability of a material to be heated by microwaves depends on its complex permittivity and loss tangent. Complex permittivity is described by equation (1):∈*=∈′−j∈″eff  (1)where real part (∈′) is a measure of how much energy from the external field is stored in the material; and the imaginary part (∈″eff) is the effective loss factor and is a measure of how dissipative the material is to an external field.
Loss tangent is defined by equation (2) and represents a ratio of energy lost and energy stored, per cycle of field oscillation.tan δeff=∈″eff/∈′  (2)
Both parameters are a function of frequency and temperature.
Rate of temperature rise within material exposed to microwave radiation is given by equation (3):DT/dt=const.*∈″eff*f*E^2/ρ*Cp  (3)where E is intensity of electromagnetic field; ∈′″eff is loss factor; f is microwave frequency; ρ is material density; and Cp is specific heat capacity of the material. Hence for a given material and microwave cavity, the rate of material heating will increase with an increase in the frequency of the microwaves and intensity of electromagnetic field squared.
Optimum transfer of microwave energy can be achieved by ensuring adequate penetration into the material and a high conversion of the microwave energy into heat. This can be achieved if the material has a moderate value of ∈′ and if the loss factor is high (i.e. a relatively high value of ∈″eff resulting in a relatively high value of tan δeff). Water is characterised by a strong ability to absorb microwave energy and transform that energy into heat.
Rocks typically comprise an aggregate of minerals in varying concentrations and at least some water, either absorbed or chemically bound. Early methods of analysing fragmentary rock samples have included irradiating the samples with microwave radiation for relatively long periods of time, generally in the order of several seconds or more. The microwave radiation differentially heats the rock fragments as observed by, for example, thermal imaging such as by an infra-red camera. Different fragments and/or areas of fragments of the rock sample are composed of different minerals and/or have different water contents. As such, these areas will increase in temperature at a different rate, and therefore to a different degree, in response to the microwave radiation. However, when constant microwave radiation is used rock fragments tend to heat very quickly throughout, reducing the detectable difference between the different components within the rock sample and forming the surface of the rock sample. The resulting infra-red thermal image tends to be essentially a blurry and indistinct depiction of the entire fragment.
In this way fragments containing some minerals which will heat and provide a generally blurry infra-red image can be sorted from those containing very little or no minerals which will heat only mildly to provide a very faint infra-red image. However, the efficacy of the sorting method is not high as it does not allow for the assessment of the level of microwave heating-associated mineral within the rock. Rather it provides a rudimentary analysis of whether the fragment contains desired minerals or not. Furthermore such methods do not provide any information about the pattern of mineral distribution in the rock fragments. For example such methods could not provide an assessment of whether the mineral deposits are located in the centre or on the surface of the rock. Due to their relative insensitivities, prior art methods have been primarily focused on separating rocks based on the different amounts of absorbed water within the rocks.
Pulsed microwave radiation has been used for inducement of micro-fractures in rock fragments to reduce the amount of energy required for subsequent crushing and comminution. The power density absorption of the microwave radiation used for this practice is generally in the range of 1,000 to 100,000 MW/m3. However, these processes with such high microwave power densities are not suitable or required for the analysis of mineral content within or on the surface of rock fragments.
Thus, it would be advantageous if a method could be provided for determining the presence of mineral within a rock more distinctly and thereby effectively enable sorting of rock fragments containing different amounts of minerals. It would also be advantageous if a method were provided that facilitates determination of the pattern of valuable mineral distribution within or on the surface of the rock fragments.
It has been surprisingly found that pulsed microwave irradiation at lower power densities than those used for the inducement of micro-fractures in rocks is superior to continuous wave microwave irradiation for analysis of the content of microwave-absorbing minerals in fragmentary samples. The use of such irradiation may allow not only the identification of fragments containing minerals but also quantification of the mineral content.